In steam generators it is common to heat the combustion air flowing to the furnace by extracting heat from the combustion gases being exhausted from the furnace. One efficient means of accomplishing this is by using a rotary regenerative air heater. With the present concern about air pollution, many plants now clean up the exhaust gases by placing an electrostatic precipitator downstream of the air heater. Because many of the noxious components contained in the exhaust gases will form highly corrosive acids, which could cause extensive damage to the precipitator and duct work if the temperature of the gas stream falls below the dew point temperature, it is important to maintain the exhaust gas temperature above this level. From an efficiency point of view, it is desirable to extract as much heat from the exhaust gases as possible. By reason of these two contrasting positions, it is desirable to maintain the temperature of the exhaust gases flowing from the air heater to the precipitator within a narrow, critical range. This presents problems when a rotary regenerative air heater is used, since there are temperature unbalances across the width of the duct downstream of the air heater. This problem is increased when more than one air heater is used, and there may be a slight difference in temperature of the gases flowing to each air heater.